This invention pertains to apparatus and methods for manufacturing semiconductor devices, displays, and the like. More specifically, the invention pertains to apparatus and methods for inspecting wafers and analogous substrates at any of various times during a wafer-fabrication procedure. Even more specifically, the invention pertains to such methods and apparatus that utilize an electron beam that is scanned over at least a portion of the wafer surface in order to reveal surficial detail of the wafer, including surface features having dimensions of 0.2 xcexcm or less.
During the manufacture of integrated circuits, displays, and the like on a semiconductor wafer or other suitable substrate, it is necessary at various steps to inspect the wafer for defects. Inspections are also required when manufacture is complete, before the wafer is cut (xe2x80x9cdicedxe2x80x9d) into individual chips or other units. Inspections are indispensable for improving yield and avoiding the shipping of defective goods.
Much wafer inspection is still performed using optical microscopes employing light to illuminate the wafer, wherein defects are detected from characteristics of light reflected from the wafer. I.e., the wafer surface is imaged using an optical microscope. The magnified image is compared with a reference pattern by means of video processing. In view of the resolution limits of light-based microscopy, these methods have increasingly limited applicability, especially with wafers on which the critical dimension is 0.2 xcexcm or less.
To obtain better resolution than obtainable with optical microscopy, electron-beam scanning apparatus have been used for inspecting wafers in the manner of a scanning electron microscope. In a conventional electron-beam apparatus, a single electron beam (focused to a point) is scanned in a raster manner over a selected portion of the wafer. When irradiated in such a manner, the wafer emits secondary electrons and backscattered electrons from the point of irradiation. The secondary and/or backscattered electrons are detected, and the presence of surficial defects can be determined from the resulting pattern of detected electrons.
Although conventional scanning-electron-beam inspection apparatus are capable of resolving detail measuring 0.2 xcexcm or less, the fact that scanning is performed using a single, narrowly focused beam results in very low inspection throughput due to the long scanning time required per wafer. Consequently, these apparatus are impractical for high-throughput wafer fabrication. Rather, they are relegated to use for supplementary defect inspection in testing situations.
This problem is circumvented somewhat simply by performing sampling inspections (in which not all wafers or only portions of wafers actually are inspected) using a scanning electron-beam apparatus. Unfortunately, this compromise results in an unacceptable amount of finished product being shipped that have major defects.
In view of the shortcomings of the prior art as summarized above, an object of this invention is to provide inspection methods and apparatus that achieve high-throughput inspection of specimens, such as semiconductor wafers and other substrates, using a charged particle beam such as an electron beam. Another object is to provide manufacturing methods, for semiconductor devices, that include such inspection methods.
To such ends, and according to a first aspect of the invention, apparatus are provided for inspecting a surface of a specimen. An embodiment of such an apparatus comprises an emitter array, first and second electromagnetic lenses, a secondary electron (SE)-detector array, and a deflector. The emitter array comprises multiple charged-particle emitters (e.g., electron-beam emitters) each configured to emit simultaneously a separate individual charged particle beam along a separate respective beam axis. The first and second electromagnetic lenses are situated downstream of the emitter array. The lenses are configured to focus simultaneously the individual charged particle beams, from the emitter array, onto respective loci on the surface of the specimen so as to cause each of the loci to emit secondary electrons. The secondary-electron (SE)-detector array comprises multiple SE-detector units each situated and configured to receive and detect secondary electrons from a respective locus on the specimen. The deflector is situated between the first and second electromagnetic lenses and is configured to deflect the charged particle beams and cause the beams to scan simultaneously respective regions on the surface corresponding to the respective loci.
In this embodiment, deflection and scanning of the individual charged particle beams can be performed by a single deflector, which imparts the same deflection and scanning to all the beams. Individual respective SE detectors detect the secondary electrons from the various loci. The SE detectors desirably are configured such that xe2x80x9ccrosstalkxe2x80x9d between the various SE detectors is negligible.
The scanning range of each charged particle beam is determined by the deflector and other components of a charged-particle-beam (CPB) optical system used to direct and focus the individual beams on their respective loci. Respective regions about the loci are inspected by scanning the respective beams within a defined range. Areas outside the regions are inspected simply by moving the specimen to within the scanning range of the charged particle beams.
The emitter array can be in one dimension (linear array). However, such an array requires that the specimen be moved continuously to obtain a two-dimensional scanning range. A two-dimensional emitter array allows less movement of the substrate, and permits (for example) inspection using a step-and-repeat scheme.
The first and second electromagnetic lenses can be configured as a symmetric magnetic doublet (SMD), which is a type of electromagnetic lens assembly used to form an image of the emitter array on the surface of the specimen. An SMD is advantageous because it minimizes the occurrence of aberrations and improves the perpendicular incidence properties of the individual charged particle beams on the specimen surface. The SMD can be a magnifying lens or 1:1 (non-magnifying and non-reducing) lens. A 1:1 lens provides excellent control of aberrations but can pose difficulties in arranging the individual SE detector units. A magnifying lens allows the emitter array to be made smaller.
The SMD further can comprise a magnification-adjusting lens.
Such a lens allows the pitch of the CPB emitters, as projected onto the specimen to be adjusted to some extent to match the pitch of, e.g., dies on the specimen. It is also possible to have various SE-detector arrays available that match the die pitch of any of several types of specimens to be inspected, wherein the array having the correct pitch can be selected for use with a particular specimen.
The charged-particle emitters of the emitter array can be in an X-Y plane (wherein the beam axes extend in a Z-direction). Alternatively, the charged-particle emitters can be displaced individually from the X-Y plane so as to correct curvature of an image collectively formed on the specimen by the charged particle beams passing through the first and second electromagnetic lenses. The most pronounced aberration in CPB optical systems using an SMD is image curvature. By shifting the respective positions of the various charged-particle emitters in the Z-direction, effects of image curvature can be reduced greatly or eliminated.
The loci typically are arrayed on the surface of the specimen with an X-direction pitch and a Y-direction pitch. Desirably, at least one (more desirably both) of the X-direction pitch and the Y-direction pitch is adjustable. Such an adjustment allows the pitch used for inspection to be adjusted to match the pitch of, e.g., dies on the surface of the specimen.
Further desirably, each SE-detector unit in the SE-detector array comprises a respective detector electrode in surrounding relationship to the respective beam axis. Each detector electrode is energized with a respective voltage. Each detector electrode also defines a respective through-hole. An SE detector is situated outside the respective detector electrode adjacent the through-hole, and charged with a voltage that is more positive than the respective voltage with which the respective detector electrode is energized. This potential profile draws secondary electrons through the through-hole to the respective SE detector. By effectively surrounding each locus with respective electrodes, secondary electrons emitted from the locus do not propagate outside the electrodes. By situating the SE detector, having a relatively high positive potential, outside the respective electrode and adjacent the through-hole, secondary electrons are drawn to the SE detector by an electrical field that extends through the hole into the space surrounded by the respective electrode. This configuration is highly favorable to capture and detection of the secondary electrons emitted from the respective locus, with minimal crosstalk between the various SE detectors. This configuration also effectively captures secondary electrons propagating in virtually any direction from the respective locus to be gathered to the respective SE detector, thereby making highly accurate defect inspection possible.
Each through-hole has a particular angular orientation about the respective beam axis. Desirably, the angular orientations of the through-holes are identical for all SE detectors of the SE-detector array. As summarized above, the electrical field produced by the potential impressed on an SE detector extends through the hole into the interior of the electrode. The respective charged particle beam is deflected by this field. If the angular orientations of the through-holes in the various electrodes were random relative to each other, then beam-deflection magnitudes and directions within the various electrodes would be different one to another. This would cause the various scanning beams to have a different profile when scanned, requiring the respective locus to be determined individually for each respective SE detector. This, in turn, would require more complex data processing and correction deflectors as required for individual beams. By disposing all the through-holes with the same angular orientation relative to the respective beam axis, the shape of the electrical field inside each electrode is the same, allowing all the beams to be deflected in the same manner. This allows for linear scanning, and the use of a single deflector to provide any needed compensation of deflection magnitude.
Each SE-detector unit desirably comprises a respective SE detector and scintillator associated with the SE detector. In such a configuration, each scintillator is connected by a light guide to a photodetector. Most of the components of the inspection apparatus (e.g., the emitter array, SMD, deflector, SE-detector array) are contained in a vacuum chamber. In an SE-detector including a scintillator and a photodetector, it is desirable that the photodetector (e.g., a photomultiplier tube) be located outside the vacuum chamber. Light is conducted from the scintillator via a light guide through a transparent window in the vacuum chamber to the photodetector. This allows the total size of components disposed inside the vacuum chamber to be minimized.
The apparatus summarized above also can include a respective scanning-position deflector for each beam. Such a deflector is configured to scan the respective charged particle beam within a respective area, corresponding to the respective locus, on the surface of the specimen. The respective scanning-position deflector can be situated upstream or downstream of the respective SE detector. Even though all the beams are deflected in the same manner by the deflector discussed above, the presence of an additional deflector for each beam allows small corrections to be applied to each beam as required, especially when performing inspections over a wide area of the specimen without moving the specimen.
According to another aspect of the invention, methods are provided for inspecting a surface of a specimen. In an embodiment of such a method, multiple separate individual charged particle beams are produced, each propagating along a respective beam axis. The individual charged particle beams are focused simultaneously onto respective loci on the specimen surface so as to cause each of the loci to emit secondary electrons. While focusing the beams, the charged particle beams are deflected simultaneously so as to cause each beam to scan a respective region corresponding to the respective locus. The secondary electrons produced from each region are detected so as to produce respective signals pertaining to the secondary electrons emitted from the regions. The signals are analyzed to produce data from which an image of the scanned regions of the surface can be formed.
The method summarized above can be used in a process for manufacturing semiconductor devices on a wafer substrate. Specifically, the method can be used for inspecting the wafer substrate at various times during wafer processing, for example. Such inspections can be performed even if the wafer substrate has die patterns having minimum linewidths of less than 0.2 xcexcm, and can be performed with good throughput. Furthermore, every die on every wafer can be inspected.
According to another aspect of the invention, apparatus are provided for inspecting a surface of a substrate on which multiple dies have been formed at an X-direction die pitch and a Y-direction die pitch. An embodiment of such an apparatus comprises an emitter array, a projection-lens system, and an SE-detector array. The emitter array comprises multiple charged-particle emitters each configured to emit a separate individual charged particle beam along a respective beam axis extending in a Z-direction. The beams collectively have an X-direction beam pitch and a Y-direction beam pitch. The projection-lens system is situated and configured to focus simultaneously the individual charged particle beams, from the emitter array, onto respective loci on the surface of the substrate so as to cause the loci to emit secondary electrons. The SE-detector array comprises multiple SE-detector units each situated and configured to receive and detect secondary electrons from a respective locus on the substrate. At least one of the X-direction die pitch and Y-direction die pitch is an integer multiple or integer fraction of the X-direction beam pitch and Y-direction beam pitch, respectively.
Whenever the beam pitch is an integer multiple of the die pitch, all of the beams are irradiated under the same conditions onto the specimen. Also, the data obtained from all of the SE detectors can be processed in the same way, allowing multiple dies to be inspected at one time. This allows inspection time to be reduced substantially with simple control.
Whenever the die pitch is a multiple of the beam pitch, one die can be inspected using multiple beams. This allows inspection to be performed at even higher throughput, further shortening inspection time.
Another apparatus embodiment includes an emitter array as summarized above. The apparatus includes multiple SE columns situated and configured to direct a respective individual charged particle beam to a respective locus on the surface of the substrate so as to cause the loci to emit secondary electrons. Each SE column comprises a respective SE-detector unit situated and configured to receive and detect secondary electrons from the respective locus. Thus, each SE column accommodates one respective beam. In the SE column, the respective beam is produced, deflected, and irradiated by a respective source, deflectors, and lenses. Multiple beams are formed by using multiple SE columns. Hence, column design is easy and simple. Also, inspection time is shortened because multiple beams are used for inspection.
Yet another embodiment of an apparatus comprises a field-emitter array comprising multiple electron emitters each configured to emit a separate individual electron beam along a respective beam axis. The apparatus also includes a projection-lens system and an SE-detector array. The projection-lens system is situated and configured to focus simultaneously the individual electron beams, from the field-emitter array, onto respective loci on the surface of the substrate so as to cause the loci to emit secondary electrons. The SE-detector array comprises multiple SE-detector units each situated and configured to receive and detect secondary electrons from a respective locus on the substrate.
In a field-emitter array, each emitter comprises a tip with a sharp point and a drawing electrode (gate) that surrounds the tip. See, e.g., xe2x80x9cField Emitter Tip Research: Present and Future,xe2x80x9d presented at the Charged Particle Beam Optics Symposium, 132nd Meeting of the Committee on the Industrial Application of Charged Particle Beams, Japanese Science Society (Oct. 29-30, 1988). By impressing a positive potential of several volts on the gate, a strong electrical field of 107 V/cm is generated in the tip. Also, the vacuum barrier (work coefficient) is reduced to about 1 nm, causing tunneling electrons to be emitted into the vacuum.
Field-emitter arrays can be manufactured using fine-processing techniques employing microlithography. Thus multiple-CPB sources can be manufactured easily and precisely. Also, as discussed in the paper cited above, stable emission currents can be realized by producing individual tips as a transistor structure.
According to another aspect of the invention, an improved SE detector is provided for use in an apparatus for inspecting a surface of a substrate (in which apparatus a charged particle beam is irradiated along a beam axis onto a beam-irradiation locus in a beam-irradiation region on the surface of the substrate to cause the beam-irradiation region to emit secondary electrons). The SE detector comprises first and second electrode members situated peripherally relative to the beam-irradiation region such that the first and second electrode members face the beam axis and each other across the beam-irradiation region. The first electrode member is charged with either a ground potential or a negative potential. The second electrode member is charged with a positive potential so as to direct the secondary electrons toward the second electrode member. Such a charging scheme directs the flow of secondary electrons toward the SE detectors.
By configuring a zero- or negative-potential member and a positive-potential member to face one another in the manner summarized above, secondary electrons are reflected from the member having the negative or zero potential and are collected on the member having a positive potential. The SE detector is placed adjacent the member on which the secondary electrons gather, thereby increasing detection sensitivity and inspection speed.
More specifically, the SE detector is situated radially outside the second electrode member. In such a configuration, the second electrode member defines a through-hole, and the SE detector is situated adjacent the through-hole and charged with a positive potential higher than the positive potential with which the second electrode member is charged. This urges secondary electrons to pass through the through-hole to the detector.
Further desirably, the SE detector is situated relative to the through-hole such that backscattered electrons emitted along respective linear trajectories from the beam-irradiation region do not impinge on the SE detector. The first and second electrode members are configured to block propagation therethrough of backscattered electrons. By blocking backscattered electrons in this manner, the backscattered electrons are not detected by the SE detector and thus do not become noise. Meanwhile, secondary electrons passing through the hole are drawn to the positive potential impressed on the SE detector and are captured by the SE detector. Even though a few backscattered electrons pass through the hole, they have high kinetic energy. Hence, the trajectory of the backscattered electrons passing through the hole is unaffected by the positive potential of the SE detector and their trajectories are unchanged. Any possible effects of backscattered electrons are avoided by placing the SE detector where it will not be struck by backscattered electrons passing through the hole. This allows for a high signal-to-noise (S/N) ratio and high-speed inspection.
In yet another embodiment of an apparatus according to the invention, multiple CPB channels are situated and configured to emit simultaneously multiple charged particle beams each along a respective beam axis to a respective locus on the substrate surface so as to cause the loci to emit secondary electrons. The channels are arranged at a channel pitch in the X-direction and the Y-direction. Each channel comprises a CPB source (e.g., electron emitter), CPB lenses, a deflector, and an SE detector. The CPB lenses focus the respective beam as a respective beam spot on the respective locus. The deflector scans the beam spot within a respective region associated with the locus on the substrate surface. The SE detector detects the secondary electrons emitted from the respective region. The apparatus also includes a pitch-adjustment mechanism associated with the channels. The pitch-adjustment mechanism is configured to change the channel pitch in at least one of the X- and Y-directions.
The substrate can be a semiconductor wafer on which multiple dies are formed at a certain pitch (die pitch). The channel pitch can be adjusted to 1/n (wherein n is an integer) of the die pitch. By making such an adjustment, the channel pitch and the die pitch can be made the same. Alternatively, for example, the die pitch can be made an integer multiple of the channel pitch, wherein each channel can inspect the same position in separate dies. Such a configuration facilitates the detection of normal and defective dies by comparing the data generated by each channel.
Desirably, each channel has an outside dimension of no greater than 30 mm in the X-direction or Y-direction. Such miniaturization is made possible in part by making at least one of the CPB lenses and deflectors electrostatic. Such miniaturization allows each die on the substrate to be inspected using a single respective channel.
Further desirably, each SE detector comprises an electrode and a scintillator. The electrode defines a through-hole and is energized with a positive charge. The scintillator is situated adjacent the through-hole but downstream with respect to the secondary electrons emitted from the respective region. The scintillator also is situated such that backscattered electrons from the respective region and propagating along a linear trajectory through the through-hole do not impinge on the scintillator. A charged particle beam incident to the substrate experiences scattering both inside the substrate and above the substrate. Hence, backscattered electrons propagate at any of various angles from the substrate surface. Secondary electrons, in contrast, are emitted only from the locus of impingement. An electrode configuration as summarized above detects only the secondary electrons that pass through the small hole in the electrode, and does not detect any backscattered electrons. This greatly improves the S/N ratio.
The light produced by the scintillator desirably is collected and focused by a condenser element. The condenser element can be connected to an optical fiber that conducts the light to a remote photodetector.
According to another aspect of the invention, methods are provided for inspecting a surface of a specimen on which multiple dies have been formed at a die pitch extending in the X- and Y-directions. In an embodiment of such a method, multiple separate individual charged particle beams are produced each propagating along a respective beam axis perpendicular to the X- and Y-directions. The beam axes are arranged at a channel pitch. The individual beams are focused simultaneously onto respective loci on the specimen surface so as to cause the loci to emit secondary electrons. While focusing the beams, the beams are scanned simultaneously over respective regions corresponding to the respective locus. Secondary electrons produced from each region are collected and detected. The channel pitch is adjusted to be 1/n of the die pitch, wherein n is an integer.
Hence, the loci can be incident at the same position in each die. With respect to any two dies, the presence or absence of defects can be determined based on the presence or absence of a difference in the signals produced by secondary electrons emitted from the loci in the two dies. The channel pitch and the die pitch can be either the same, or the die pitch can be an integer multiple of the channel pitch. Each channel can be made incident to the same locus in dies at two different locations.
Alternatively, at least three beams can be made incident at a same position in separate respective dies on the specimen. In such a configuration, the signals produced by secondary electrons emitted from the separate respective dies are compared to determine any difference in the signals indicating a difference in the respective dies.
By comparing the signals from three or more locations, it can be determined in which die a defect is located. In other words, if the signals from two locations are the same and the signal from the third location is different, then it can be determined that the defect actually is in one location.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.